We propose to investigate the effects of a lesion of the entorhinal cortex on synaptic reorganization in the dentate gyrus. Our results obtained to date indicate removal of entorhinal cortical input serves as a stimulus to those remaining fibers to grow into the field of dying terminals and rewire the neuron. We will determine if an adult entorhinal lesion allows the contralateral entorhinal cortex to establish a new projection to the dentate gyrus which has lost its cortical input. We will also determine if associational projections reorganize their connections in the dentate gyrus after an adult lesion. We will gather both anatomical and neurophysiological data on the state of both of these projections after lesion. Possible alterations in gamma aminobutyric acid interneurons will be assessed by microchemical analyses. We will undertake experiments on the mechanism of reactive synaptogenesis after lesions. We will attempt to determine the role of the postsynaptic density in reinnervation. We will also explore the possible role of inducer molecules in the degeneration-reinnervation-regeneration process. Our general objective is to establish the dimensions and principles of function plasticity after lesions.